This invention relates to an electrically conductive polyimide film having good physical properties and uniform electrical conductivity, prepared by coextruding at least two aromatic polyamic acid solutions, one of which contains a conductive filler, to form a conductive multi-layer polyimide film, e.g. a two-layer or three-layer polyimide film.
Conductive polyimide films containing carbon particles are well-known in the art. For example, Canadian Patent 708,896 discloses an electrically conductive polyimide film prepared by blending conductive carbon particles, e.g. carbon blacks, in the precursor polyamic acid; shaping the carbon particle containing polyamic acid into a film; and then thermally converting the polyamic acid into a polyimide film containing the carbon particles.
U.S. Pat. No. 5,075,036, issued to Parish et al on Dec. 24, 1991, discloses an electrically conductive polyimide film containing carbon black and graphite particles uniformly dispersed therein and having a surface resistivity from about 130 to 1.times.10.sup.10 ohms/square.
In order to obtain surface resistivities of less than 150 ohms/square in such conductive polyimide films, relatively large amounts of conducting fillers such as carbon black, metal powder and/or graphite are normally required. Typically, the concentrations of conductive filler that are required to obtain low resistivities are high enough to weaken the structure of the polyimide substrate. For example, a 1 mil thick unfilled polyimide film has the following properties:
Surface resistivity=&gt;10.sup.16 ohms/square PA1 Elongation to break=85% PA1 Elmendorf tear=20g/mil PA1 MIT fold endurance=35,000 cycles PA1 Surface resistivity=150 ohms/square PA1 Elongation to break=9.7% PA1 Elmendorf tear=1.4g/mil PA1 MIT fold endurance=300 cycles
A 1 mil thick polyimide film containing 30% by weight of carbon black filler particles typically has the following properties:
Therefore, high concentrations of carbon black seriously degrade the physical properties of the conductive polyimide film.
Conversely, lower concentrations of conductive filler particles provide better film properties, but the film thicknesses required to achieve low surface resistivities are quite large, as can be seen from the following relationship: ##EQU1##
Another problem with using low conductive filler concentrations is the influence of the polyimide polymer matrix on the conductive particles. Film surface resistivities can be very non-isotropic, i.e. non-uniform, when measured either directionally or as a function of the test frequency.
Polarity is defined as the difference in electrical resistivities measured in the longitudinal (MD) and transverse (TD) film directions divided by the average resistivity, i.e. ##EQU2##
At lower conductive filler concentrations, polarity is very strongly influenced by the degree of stretching or orientation experienced by the film. Consequently, it is possible for a film to have either positive or negative polarity. Although zero polarity is preferred, generally plus or minus eight % is acceptable for most applications.
On the other hand, the resistivity-frequency relationship, defined as taper and being more a function of additive concentration, is not as readily influenced by film orientation. Taper is defined as the differences in resistivities, when measured at 2 and 18 gigahertz, divided by the average resistivity, usually measured at 10 gigahertz. Thus, a conductive polyimide film having an average surface resistivity of 400 ohms/square measured at 10 gigahertz, a maximum resistivity of 480 ohms/square at 10.sup.18 gigahertz and a minimum resistivity of 380 ohms/square at 2 gigahertz has a taper of 0.25 or 25%. As with polarity, zero taper is preferred, but a range of plus or minus 18% is acceptable for most applications.
The object of the present invention is to provide a conductive multi-layer polyimide film prepared by coextruding a conductive carbon filled polyimide layer and one or more unfilled polyimide layers having high conductivity, good taper and polarity while still retaining the good physical properties of the polyimide.